What angle should a solar array be tilted to get maximum energy?

For 25 degrees north, the array should point to the sun in the same way, with the north end raised 25 degrees (for a northern site). Since the sun is so far away, the rays come to Earth each approximately parallel to the other rays. So to point the array parallel to the equatorial plane, the array must tipped or tilted towards the equator. How much? By the latitude angle of 25 degrees. The north edge is tilted up. That angle will point the normal vector (90 degrees to the array surface) at the sun for solar noon on the equinoxes. The rest of the year, it's too much or too little, but the offset angle averages to zero error.

For South-dwellers of the Southern Hemisphere, say 16 degrees south, the array is still pointed parallel to the equatorial plane, but the sun is north. A 16 degree tilt of the south edge up from horizontal makes the array normal parallel to the equatorial plane.

Essentially, the tilts average out the received energy over a year. Suppose you wanted more energy. You could drill more bolt holes and move the array perhaps three times a year.

For more, read Boyle, pages 45-48. This is the most important solar concept.